1. Field of the Invention
This invention relates to manufacture of sodium dithionite and particularly relates to preparation of sodium dithionite in an aqueous slurry form having adequate stability for industrial bleaching operations. More particularly, it relates to such manufacture directly from commercially prepared solutions of sodium dithionite.
2. Review of the Prior Art
Sodium dithionite, commonly termed sodium hydrosulfite and less correctly sodium hyposulfite, is a powerful reducing agent that has long been used for bleaching, particularly for bleaching textiles and wood pulps such as groundwood and semichemical pulps. However, the stability of sodium dithionite has long been a problem.
Sodium dithionite has usually been manufactured by significantly different processes that are alternatively based on zinc dust, sodium formate, sodium borohydride, or sodium bisulfite (electrolytic). The products from these processes are herein respectively identified as zinc-derived, formate-derived, borohydride-derived, or electrolytically-derived sodium dithionite. Because the zinc process produces zinc dithionite which is no longer ecologically acceptable, the zinc dithionite is converted to sodium dithionite by adding sodium hydroxide or sodium carbonate, whereby zinc hydroxide or zinc carbonate is precipitated and removed by filtering. The filtrate containing the sodium dithionite is termed sodiation liquor.
Sodium dithionite from any of these four processes is potentially available on a commercial basis in the form of powders, solutions, or slurries. When anhydrous sodium dithionite crystals are dissolved under either aerobic or anerobic conditions, to make a large quantity of aqueous solution, the resulting solution cannot be stored for use over a long period of time. Due to hydrolytic decomposition at the natural pH of a sodium dithionite solution, decomposition will proceed rapidly from that point by self-propagation because the decomposition products create an acidic condition which accelerates the decomposition.
Under normal storage conditions, commercial grades of powdered sodium dithionite can safely be held without appreciable decomposition for long periods of time, if kept dry. But if moisture reaches sodium dithionite in confined places, such as stored drums or storage bins, exothermic decomposition reactions can cause self-ignition of the decomposition products and even explosions if the sodium dithionite is confined in a gastight space, such as a drum. Moreover, production of heat by this decomposition can release large volumes of sulfurous gases and can cause fires which are very difficult to extinguish.
More specifically, it has been reported that dry sodium dithionite can decompose in 46-200 minutes by adding 3% of water onto its surface at room temperature in an insulated container, and such decomposition can occur in one to two minutes by adding 0.7% of water onto its surface at 133.degree. C. Such contact with moisture can occur because of moisture from the air, residual moisture from incomplete drying of a product during manufacture, or accidental wetting of the material. Further, decomposition and fires can be caused, without water contact, by heating to temperatures of 135.degree.-190.degree. C.
As an example of many attempts to avoid these difficulties, U.S. Pat. No. 3,054,658 proposes the addition of 0.1-45% by weight of a sodium or potassium salt of a carboxylic acid (e.g., formic acid to stearic acid) and a benzoic acid, which may be substituted in the ring by an amino, hydroxy, or methyl group (e.g., p-amino benzoic, salicylic, and o-, m-, p-toluic acid) to form a stabilized mixture which at the one percent addition level does not begin to decompose after more than one hour as compared to 1.5 minutes without an added stabilizing agent.
It is industrially significant that while powdered sodium dithionite is dissolving to form a bleaching solution, it tends to decompose at an appreciable rate. Moreover, the commercially prepared solutions of sodium dithionite also tend to decompose during the five to seven days that are required for transportation from the place of manufacture to a textile or pulp mill and for onsite storage within the mill. Such decomposition can proceed by aerobic reactions, or by anaerobic reactions when air is completely excluded.
As an example of a large number of attempts to protect sodium dithionite solutions during transportation and storage thereof, U.S. Pat. No. 3,985,674 provides sodium dithionite bleaching solutions for groundwood pulps which require no additional chemicals for pH adjustment prior to direct application to the pulps, by using at least four additives selected from the following: a chelating agent, zinc dithionite, zinc sulfate, sodium carbonate, sodium hydroxide, sodium tripolyphosphate, sodium phosphate, and sodium metaborate. Chelating agents include nitrilo triacetic acid (NTA) trisodium salt and ethylene diamine tetraacetic acid (EDTA) tetrasodium salt. The solutions are cool stable at 50.degree. F. for at least five days with less than 5% decomposition of the sodium dithionite.
It is an economic reality that powdered sodium dithionite is expensive to manufacture. It is also true that its pulp-bleaching purchasers, who must store it under the dark, warm, and humid conditions that prevail in a pulp and paper mill, have to expend care and expense to utilize oldest drums first, to watch for and utilize machine-damaged and rust-damaged drums without delay, and to provide fire-fighting protection that might otherwise be unnecessary. In brief, powdered sodium dithionite is not the ideal form for this compound under some industrial conditions.
It is furthermore another economic reality that commercially available solutions of sodium dithionite are expensive to transport because they are typically at concentrations of 5-13.5%, preferably 12-13.5%, when combined with suitable additives. Thus, the transport of about seven times as much water as product tends to cause the sale of this commodity to become distance-dependent. In consequence, slurries have seemed to offer an inviting means to avoid or at least to minimize the cost and storage difficulties associated with solution forms of sodium dithionite, without decreasing the convenience that a purchaser derives from solutions.
However, the economical preparation, stabilization, handling, and shipping of such slurries is not simple. In fact, after considering the variety of processes that are available for manufacturing sodium dithionite, including the indigenous by-products, crystal structures, and the like, the complexities of the concept are readily appreciated. Moreover, slurries have not been as widely investigated nor as commercially utilized as other forms of sodium dithionite.
U.S. Pat. No. 3,536,445 describes a process for making sodium dithionite from sodium-zinc alloy by initially producing zinc dithionite and then converting it to sodium dithionite by adding caustic soda. After removal of the zinc hydroxide by filtration, the dihydrate of sodium dithionite is "salted out" of the mother liquor with sodium chloride and alcohol to form a slurry.
U.S. Pat. No. 3,804,994 gives some stability storage data for 30% slurries (18.5% formate-derived and 11.5% zinc-derived sodium dithionite) containing 1-8% caustic soda (dithionite basis). Tests showed that these slurries required frequent agitation to prevent caking and handling difficulties. None of the slurries survived for 20 days at 67.degree. F. before exceeding 10% decomposition. Most survived the 30-day test at 50.degree. F., and all were successful at 35.degree. F. for thirty days, with most showing less than five percent decomposition for the test period. It was found that a 10% dithionite solution was more easily handled than a solution containing 15% or more of sodium dithionite which developed needle-like crystals of sodium dithionite dihydrate which were difficult to redissolve. Moreover, the stability was inversely related to the dithionite concentration.
U.S. Pat. No. 3,839,217 shows that by reducing the particle size of the sodium dithionite crystal and/or introducing a suspending or thickening agent into a liquid containing the crystals, such as alcoholic brine, it is possible to form a fluent, homogeneous, pourable dispersion of the solid dithionite particles which is chemically and physically stable for long periods of time, provided that a material, such as the salt in the brine and/or an alcohol, be present which suppresses the dissolution of the dithionite. The majority of the particles should be about 0.6-0.8 micron. Methylcellulose, polyvinyl alcohol, and other common thickening, dispersing, or suspending agents can be used.
U.S. Pat. No. 3,839,218 provides a method for maintaining a dispersion of crystalline zinc or alkali metal dithionite hydrate by continuous or periodic mechanical agitation so that the crystals can be stored for long periods without decomposition, the dispersing medium being aqueous or non-aqueous and containing a material which suppresses dissolution of the dithionite solids. The pH of the liquid must be at least 6.5, the viscosity of the dispersion must be below about 50,000 centipoises, and the suppressing material may be a water-soluble organic compound or a saturated brine or mixtures thereof.
In general, the slurries of U.S. Pat. Nos. 3,839,217 and 3,839,218 are potentially economically feasible for processors which produce sodium dithionite dihydrate crystals in slurry form by "salting out" techniques before filtering, dehydrating, and drying to produce anhydrous sodium dithionite, such as by the method of U.S. Pat. No. 3,936,445. However, it is at least inconvenient when anhydrous sodium dithionite is produced directly by precipitation from an aqueous methanol solution at a temperature above the dehydration point of a hydrated sodium dithionite, such as by the method of U.S. Pat. No. 4,127,642. In order to produce a slurry from such anhydrous sodium dithionite, it is necessary to dissolve the anhydrous powder in water and then to add a suppressant, an organic liquid, a buffer, and/or a suspending agent. Clearly, following this route to produce a slurry is expensive and awkward.
Therefore, because the formate process produces anhydrous sodium dithionite directly, shipping and sale of the formate-derived powder in air-and-moisture-free drums is preferable. But in the other processes, such as the zinc-derived process, wherein the dithionite is produced as an aqueous solution, additional steps are necessary to produce anhydrous sodium dithionite, such as: (1) treating the sodiation liquor with sodium chloride and ethanol to effect crystallization of the dihydrate and (2) drying the dihydrate. This salting-out procedure has been practiced for years in the zinc-derived process. However, it is preferable for many reasons to market the sodiation liquor, which contains sodium dithionite at 16-18% concentration, as a solution by diluting it to about 13% Na.sub.2 S.sub.2 O.sub.4 and by adding a chelating agent and NaOH as a stabilizer. Thus, an economic impetus to create a process that can produce a slurry directly from the sodiation liquor is provided by the economics of shipping water in such solutions.
A continuous process for producing anhydrous sodium dithionite in a single stage is disclosed by Dulepov et al in Zhurnal Prikladnoi Khimii, Vol. 49, No. 5, pp 1135-1137, May 1976 (Russian). In this process, a sodium dithionite solution containing about 28% NaCl (dithionite basis) is evaporated under vacuum at a temperature above the point of conversion of the dihydrate into the anhydrous salt, so that the product separates out into a solid phase in the form of anhydrous crystals. A crystallization temperature of 58.degree. C. (136.degree. F.) is suitable. No stability information appears to be given except a storage decomposition rate of 0.0375% per minute at 58.degree. C. (136.degree. F.) within the evaporator.
In general, the prior art has prepared slurries of sodium dithionite and has merely added caustic soda to provide a sufficiently high pH but has not otherwise investigated the stabilities of dithionite slurries. Thus, while making such an investigation, it has been discovered that even in the presence of caustic soda and a chelator, stabilities were sometimes excellent and sometimes very poor. The borderline value for an acceptably stable slurry is herein defined as no more than 0.65%/day at a storage temperature of 35.degree.-40.degree. F. There is consequently a need for a method that will provide reproducably stable dithionite slurries, and there is further a need for a method that will produce such stable slurries without having to salt out the crystals.